As one of large-capacitance capacitors used in various electronic devices, a type of solid electrolytic capacitor wherein a solid electrolytic capacitor element having a dielectric oxide film, a semiconductor layer and an electrode layer stacked on a sintered body of conductive powder and the whole is encapsulated with resin.
A solid electrolytic capacitor is fabricated by encapsulating an element consisting of a sintered body of conductive powder such as tantalum having micropores inside it as one electrode (conductor), a dielectric layer formed on the electrode, the other electrode (generally, semiconductor layer) formed on the dielectric layer and another electrode layer formed on the said other electrode. When volumes of conductors are the same, the smaller the size of the micropores and the larger the number of the micropores, the larger the inside surface area of a conductor and therefore, the larger the capacitance of a produced capacitor from the conductor.
Recently, in a solid electrolytic capacitor, low ESR (Equivalent Series Resistance) is required. To meet this, an electroconductive polymer is always employed as an inner semiconductor layer. Such a semiconductor layer is formed by chemical polymerization method or electrolytic polymerization method. As one example, a semiconductor layer is formed by immersing a conductor having a dielectric layer thereon in two solutions separately prepared, alternately, one containing oxidant and dopant and the other containing monomer, except for anode lead of the conductor.
A semiconductor layer is formed by pure chemical reaction without operation of applying current (i.e., solution reaction, vapor phase reaction or combination thereof), by a method involving applying of current or by combination of these methods.
Examples of method involving applying of current include a method where a semiconductor layer is formed on the dielectric layer by applying current to a cathode plate provided in a solution for forming a semiconductor layer, with a conductor or an external electrode placed in contact with or in the vicinity of the conductor being used as anode. Methods of applying current include constant-voltage approach, constant current approach and combination thereof. For the purpose of stable formation of semiconductor layer in a case where current is applied to multiple conductors simultaneously, constant current approach is preferred.
As cathode plate, a plate of stainless-steel, platinum, tantalum or the like, less corrodible by a liquid in which current is passed is used. For example, as described in Japanese Utility Model Application Laid-Open No. H05-36267 (Patent Document 1), a group of conductors fixed to an anode plate so that the fixed conductors can be prominent like comb teeth is immersed in a bath for forming a semiconductor layer and a cathode plate is provided so that the cathode faces the anode. In this document, metal plates having on the surface a protrusion at each position corresponding to each of the conductors for the purpose of controlling electric fields are used. In conventional techniques including this example, surface area of a cathode metal plate is much smaller that the total surface area of a group of conductors having micropores on the surface.    [Patent Document 1] Japanese Utility Model Application Laid-Open No. H05-36267